1. Field of the Invention
The present invention relates generally to seals for use in fuel cell applications. More particularly, the present invention relates to a composite seal comprising a multiple layer elastomeric construction for providing superior sealing performance and durability in proton exchange membrane (PEM) fuel cell applications.
2. Description of the Prior Art
A PEM fuel cell is assembled from multiple component layers, the essential component layers including a reactive membrane and a plurality of gas diffusion layers (GDL) making up the membrane electrode assembly (MEA) and opposing bipolar plates (one for enclosing the anode side and another for enclosing the cathode side of the cell). The MEA is interposed between the anode and cathode bipolar plates. Additional components may be used to help draw hydrogen fuel and oxygen gas into the fuel cell assembly and to conduct the electrical current generated by their corresponding interaction.
The use of PEM fuel cells as a means for generating electrical current is achieved through a controlled electrochemical reaction, driven by the MEA, between the hydrogen fuel and oxygen gas. More specifically, hydrogen is permitted to flow into the fuel cell on the anode side and is catalytically split into hydrogen ions (i.e., protons) and electrons. The ions permeate across the MEA to the cathode side, while the freed electrons flow through an external circuit coupled to the anode and cathode sides of the fuel cell to drive a load. As the hydrogen ions permeate through the MEA, oxygen is permitted to flow on the cathode side to combine with the electrons traversing the circuit load and the hydrogen ions permeating through the MEA. The recombination of these elements results in the formation of water and heat.
To ensure proper containment of the aforementioned electrochemical reaction, various gasket constructions are used to seal the periphery of the MEA interposed between anode and cathode bipolar plates. Many of these gasket constructions are known to utilize silicone based elastomers. Whereas silicone based elastomers are typically used in low temperature (i.e., less than 180° C.) fuel cell applications, their use in high temperature fuel cell applications is problematic due to their vulnerability to deterioration over time and subsequent interaction with a fluorene constituent, as well as the high pH values, commonly associated within a fuel cell environment. The use of silicone in low temperature fuel cell applications can be problematic as well in that its use in low temperature fuel cell applications has the potential for long term permeation of low molecular weight gases and materials through the silicone body due to its inherently porous molecular structure. Despite the foregoing, silicone remains the current standard even though in some cases it may not be the preferred sealing material.
Fluoroelastomer fluoropolymers, having a more resilient structure, are preferred for sealing applications. Fluoro materials, however, generally do not possess durometer values less than 55 Shore A, which presents an additional problem with respect to providing the necessary compliance for sealing anode and cathode bipolar plates to an adjoining MEA without resulting in undue stress on components of the fuel cell assembly. This is particularly a concern when dealing with bipolar plates comprised of inherently brittle graphite constructions.
Accordingly, it is desirable to provide an improved sealing solution, said sealing solution providing the necessary chemical and thermal resistance properties, as well as the necessary compressive compliance properties, needed for use in both low and high temperature yielding fuel cell applications. In addition to providing a resilient and structurally compliant sealing solution, it is also desirable to minimize misalignment or sealing vulnerabilities commonly associated with loose die-cut or free molded gaskets by providing a more integral sealing solution.